The present invention relates to the field of microfluidics and, more particularly, to using polyelectrolyte complex films to control the magnitude and direction of electroosmotic fluid flow within microfluidic channels.
In the field of microfluidics, liquid is moved through passages of micrometers dimension. These passages may be isolated or interconnected, and may be part of a larger scheme for performing fluid manipulation for the purposes of analysis or synthesis, for example, in a lab-on-a-chip apparatus. Microfluidic flow may be employed for analytical separations, or for moving reagents and samples through different reaction and/or detection zones. Various means are available for causing liquid to flow in microfluidic systems. For example, flow may be caused by pressure differences between ends of a microfluidic channel. Alternatively, flow may be caused by electroosmosis. Electroosmotic flow, EOF, of liquid through a microfluidic passage requires a net immobile charge on the interior surface of said passage, in contact with the liquid. The immobile surface charge is balanced by mobile counterions of the opposite charge from the fluid, usually water. Depending on the sign of the surface charge, the microfluidic channel thus contains an excess of mobile cations or anions. Under the influence of an electric field, imposed along the microfluidic channel by electrodes (anode and cathode) in contact with the fluid, there is a net migration, towards one end of the microfluidic channel, of anions or cations. Since these ions are solvated, they drag solvent molecules with them, causing a net flow of solvent within the microfluidic channel. If the surface charge on the interior of the microfluidic channel is negative, the channel will comprise excess cations, and fluid flow will be towards the negative electrode (the “normal” direction). If the surface charge is positive, the microfluidic channel will comprise excess anions, and net flow will be towards the positive electrode (the “reversed” direction). Since the direction and velocity of EOF is critically dependent on both the sign and magnitude of the surface charge on the interior of the microfluidic channel, there is a need for a means to produce a well-defined, stable surface charge within microfluidic systems.
EOF is advantageous compared to pressure-driven flow because the flow profile in EOF is more plug-like, whereas the flow profile in pressure-driven flow is parabolic. A plug-like flow profile leads to less dispersion of material flowing through a microfluidic channel. In addition, EOF is simple to implement on a chip, requiring only the placement of electrodes.
Species to be transported within microfluidic channels by EOF are swept along by the movement of the liquid. If a species in a microfluidic channel is charged it also experiences a force due to the electric field imposed on the channel. Motion due to the charge of a species within a liquid or gel under an applied field is known as electrophoresis. A charged species within a microfluidic channel therefore experiences displacement due to EOF and electrophoretic flow simultaneously. The differential speed of migration between two or more charged species as they travel down a microfluidic channel permits these species to be separated from each other. In some cases, it is required that a charged species moves by electrophoresis only, and therefore the net charge on the microfluidic channel must be zero to completely suppress EOF.
Capillary zone electrophoresis, CZE, is one embodiment of a microfluidic channel where the channel is formed by a length of capillary tubing, typically fused silica, of internal diameter in the range 10 micrometers to 200 micrometers (see Rose and Jorgenson, U.S. Pat. No. 4,936,974). The length of the capillary is typically a few tens of centimeters and the exterior of the capillary is usually coated with a polymer, such as a polyimide, to impart physical durability. The capillary is filled with electrolyte, typically buffered aqueous solution, and the two ends are dipped into a reservoir. A voltage, typically in the kilovolts range, is applied along the length of the capillary by electrodes also dipping into the reservoir. The electric field, measured in volts per centimeter, is a critical parameter, so shorter lengths of capillary require lower applied voltage. What may be considered the first apparatus for CZE was described by Jorgenson and Lukas (see J. Jorgenson and K. D. Lukas, Analytical Chemistry, 53, 1298, (1981)).
Polyelectrolytes are macromolecules comprising a plurality of charged repeat units. Amorphous complexes may be formed by contacting solutions of polyelectrolytes bearing opposite charges. The driving force for association, or complexation, of polyelectrolytes is multiple ion pairing between oppositely charged repeat units on different molecules.
Recently, thin films of polyelectrolyte complexes have been prepared using polyelectrolytes which are alternately deposited on a substrate or substratum. See Decher and Schlenoff, Eds., Multilayer Thin Films—Sequential Assembly of Nanocomposite Materials, Wiley-VCH, Weinheim (2003); Decher, Science, 277, 1232 (1997). Decher and Hong (U.S. Pat. No. 5,208,111) disclose a method for a buildup of multilayers by alternating dipping, i.e., cycling a substrate between two reservoirs containing aqueous solutions of polyelectrolytes of opposite charge, with an optional rinse step in polymer-free solution following each immersion. Each cycle adds a layer of polymer via ion pairing forces to the oppositely-charged surface and reverses the surface charge thereby priming the film for the addition of the next layer. Films prepared in this manner tend to be uniform, follow the contours and irregularities of the substrate, and are typically between about 10 nm and about 10,000 nm thick. The thickness of a film depends on many factors, including the number of layers deposited, the ionic strength of the solutions, the types of polymers, the deposition time, the solution pH, the temperature, and the solvent used. Although studies have shown that the substantial interpenetration of the individual polymer layers results in little composition variation over the thickness of a film, such polymer thin films are, nevertheless, referred to as polyelectrolyte multilayers (PEMUs).
Though recently developed, PEMUs are being used in a wide variety of fields including light emitting devices, nonlinear optics, sensors, enzyme active thin films, electrochromics, conductive coatings, patterning, anticorrosion coatings, antistatic coatings, lubricating films, biocompatibilization, dialysis, and as selective membranes for the separation of gaseous and dissolved ionic species. See Fou et al., J. Appl. Phys., 79, 7501 (1996); Decher et al., J. Biosens. Bioelect. 9, 677 (1994); Sun et al., Macromol. Chem. Phys. 197, 147 (1996); Onda et al., Biotech Bioeng. 51, 163 (1996); Lvov et al., J. Am. Chem. Soc. 120, 40733 (1998); Laurent et al., Langmuir 13, 1552 (1997); Stepp et al., J. Electrochem. Soc. 144, L155 (1997); Cheung et al., Thin Solid Films 244, 985 (1994); Hammond et al., Macromolecules 28, 7569 (1995); Huck et al., Langmuir 15, 6862 (1999); Stroeve et al., Thin Solid Films 284, 708 (1996); Levasalmi et al., Macromolecules 30, 1752 (1997); Harris et al., Langmuir 16, 2006 (2000); Krasemann et al., Langmuir 16, 287 (2000); Harris et al., J. Am. Chem. Soc. 121, 1978 (1999); and Harris et al., Chem. Mater. 12, 1941 (2000).
Polyelectrolyte complexes are known to moderate interactions with biological systems, usually with the purpose of rendering an article or object inert to biological activity. That is, a coating of polyelectrolyte complex does not elicit undesirable inflammation or immune responses. Fine tuning of protein adsorption at the solid/liquid interface is critical in certain areas of materials science and biomedical engineering. Systems for delivery or biosensors, for example, bear modified surfaces designed to enhance or minimize protein adsorption. The latter goal is generally desirable for blood-contacting devices, chromatographic supports, contact lenses, and immunoassays, to name a few. Due to their ease of use and water compatibility, PEMUs have been investigated as surface-modifying agents for protein interactions (see Ladam, G.; Gergely, C.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P.; Cuisinier, F. J. G. Biomacromolecules 1, 674 (2000)). For example, polyelectrolyte complexes have been coated on Islets of Langerhans, an insulin-producing biological apparatus, to make them more acceptable when implanted in vivo (see O'Shea and Sun, Diabetes 35, 953 (1986) and Goosen et al. U.S. Pat. No. 4,673,566 (1987)). In another example, an ocular contact lens treated with a polyelectrolyte complex improves the properties of the lens (see Ellis and Salamone, U.S. Pat. No. 4,168,112 (1979)). Winterton et al. (U.S. Pat. No. 6,451,871 (2002)) disclose a method of making polyelectrolyte complexes on the surface of a contact lens by the multilayering method.
Protein adsorption is driven by the net influence of various interdependent interactions between and within surfaces and biopolymer. Possible protein-polyelectrolyte interactions can arise from 1) van der Waals forces 2) dipolar or hydrogen bonds 3) electrostatic forces and 4) hydrophobic effects. Given the apparent range and strength of electrostatic forces, it is generally accepted that the surface charge plays a major role in adsorption. However, proteins are remarkably tenacious adsorbers, due to the other interaction mechanisms at their disposal.
The use of thin films of polyelectrolyte complex for coating the interior surfaces of capillaries for CZE has been disclosed by Katayama and Ishihama (U.S. Pat. No. 6,586,065) and by Schlenoff and Graul (U.S. Pat. No. 6,402,918). Furthermore, Graul and Schlenoff (see Analytical Chemistry, 71, 4007 (1999)) describe how, in the analytical separation of proteins, the surface charge of the thin film of polyelectrolyte complex may be selected to have the same charge as the proteins being separated, thus preventing adsorption of the protein to the capillary. Preventing adsorption is considered advantageous in analytical CZE because the separation efficiency is greatly improved.